The present invention relates to a method of extracting a charged particle beam from an accelerator, and an accelerator capable of carrying out the method.
In a conventional circular accelerator, accelerated electrons or ions are extracted from the accelerator, to be used for physical experiments, medical purposes, or others. Further, as discussed on pages 53 to 61 of the AIP Conference Proceedings No. 127 (1983), the third order resonance or half integer resonance of betatron oscillation is used for extracting the accelerated electrons or ions from the accelerator.
FIG. 1 shows an example of the conventional accelerator. Referring to FIG. 1, charged particles injected from an inflector 15 circulate stably around an equilibrium orbit 1 (that is, closed loop, around which charged particles can revolve stably) while making betatron oscillations, with the aid of bending magnets 3, focusing quadrupole magnets 5 and defocusing quadrupole magnets 7. Further, the charged particles acquire energy when they pass through an accelerating cavity 8. In such an accelerating process, the tune of charged particles, that is, the number of betatron oscillations per one revolution of a charged particle along the equilibrium orbit, is usually set, in both of horizontal and vertical directions, to an integer .congruent.1/4, with the aid of the focusing magnets 5 and the defocusing magnets 7.
The betatron oscillation of third order resonance or the half integer resonance are excited, when the accelerated, charged particle beam is extracted from the accelerator. In order to extract the beam from the accelerator in the horizontal or vertical direction, the tune in the horizontal or vertical direction is adjusted in the following manner with the aid of the quadrupole magnets 5 and 7. That is, in the case of the half integer resonance, the tune is made to approach an integer .+-.1/2. While, in a case where the third order resonance is used, the tune is made to approach an integer .+-.1/3. In this state, the half integer resonance is generated by exciting an octupole magnet which is previously installed in the accelerator, or the third order resonance is generated by exciting a (for example, sextupole magnet 5 of FIG. 1). Thus, the betatron oscillation of the half integer resonance or the third order resonance are excited, and the amplitude of betatron oscillations of charged particles over a stable limit increases to a great degree. Further, in order to prevent the charged particle beam in this state from colliding with a duct, a pair of dipole, bump magnets 10 and 11 are excited to shift that portion of the equilibrium orbit which exists in the neighborhood of a deflector 13 for extraction, to the deflector side, thereby forming a bumped orbit 12 (that is, a locally distorted orbit).
The behavior of the beam at this time will be explained below, with reference to phase spaces shown in FIGS. 2A and 2B. FIGS. 2A and 2B show phase spaces in the initial and last stages of beam extraction at the deflector due to the third order resonance, respectively. In FIGS. 2A and 2B, the abscissa indicates the displacement x of the beam in the horizontal direction, and the ordinate indicates the gradient of orbit x' of the beam (where x'=dx/ds, and s indicates a distance in the circular orbit direction). In the case where the third order resonance is used, a triangular separatrix (that is, stable limit) such as the triangle .DELTA.ABC of FIG. 2A is formed. Further, in this case, the tune is nearly equal to an integer .+-.1/3. Accordingly, a charged particle is put in substantially the same state position with the phase space, after three revolutions along the equilibrium orbit. That is, each charged particle can be put in three states at an extraction point. In FIGS. 2A and 2B, three directed curves indicate the transition of three states of each charged particle, on the outside of the separatrix. For example, a charged particle in the state a.sub.1 of FIG. 2A is put in a state b.sub.1 after having made one revolution along the equilibrium orbit, is put in a state c.sub.1 after having made two revolutions, and is put in a state a.sub.2 after having made three revolutions. Finally, the charged particle reaches the deflector 13 which is spaced apart from the equilibrium orbit a distance x.sub.d, and thus is extracted from the accelerator
A portion of charged particles revolving along the equilibrium orbit is large in amplitude of betatron oscillation, and another portion of the charged particles is small in the above amplitude. In order to extract charged particles gradually from the accelerator, the charged particles are extracted in order of amplitude of betatron oscillations. Accordingly, the separatrix is made relatively large in the initial stage of extraction as shown in FIG. 2A, and is made small with time. This operation is performed by a supplementary quadrupole magnet 14 shown in FIG. 1, or by a supplementary coil mounted on each of a pair of quadrupole magnets 5 and 7. That is, the tune approaches an integer .+-.1/3 with the aid of the supplementary quadrupole magnet 14 or the supplementary coil mounted on each of a pair of quadrupole magnets 5 and 7, and thus the separatrix is reduced to the triangle .DELTA.A'B'C' of FIG. 2B.
As can be seen from the trajectory of each charged particle shown in FIGS. 2A and 2B, when the charged particle beam is extracted from the accelerator in the above-mentioned manner, the gradient of orbit of the extracted beam in the initial stage of extraction having a large separatrix such as the triangle .DELTA.ABC of FIG. 2A is different from the gradient of orbit of the extracted beam in the last stage of extraction having a small separatrix such as the triangle .DELTA.A'B'C' of FIG. 2B, and thus there arises a problem that it is impossible to extract a strong, charged particle beam.
Recently, it is required to make the accelerator small in size and to use the accelerator for industrial or medical purpose. However, when the fine adjustment of the tune for extracting charge particles is made by the supplementary quadrupole magnet 14 in a small-sized accelerator, interference will be generated between the supplementary quadrupole magnet 14 and other constituent elements of the accelerator. Thus, an accelerator can not be made small in size.